1. Field of the Invention
The present invention relates to a polyimide alignment film based on pyromellitic dianhydride and a bis(4-aminophenoxy)aromatic compound and to a liquid crystal display device using such an alignment film.
2. Description of the Prior Art
Liquid crystal display (LCD) devices have become increasingly important in displays which require very low consumption of electrical power or where the environment dictates a lightweight, planar, flat surface. Thus, LCD's are used in display devices such as wristwatches, pocket and personal computers, aircraft cockpit displays, etc.
In its simplest form, a liquid crystal display device consists of a liquid crystal layer with opposite sides, a set of electrodes on either side of the liquid crystal layer and an alignment polymer layer between each set of electrodes and the liquid crystal layer. Alignment of the liquid crystal molecules occurs at a certain angle, referred to as the tilt angle, with respect to the plane of the inside of two substrates, e.g. glass plates, plastic sheets, quartz plates or others, which support the electrodes. The inside of the substrates have coatings of sets of transparent electrodes (electrical conductors), usually indium-tin oxide (ITO). The sets of electrodes are patterned, e.g. by etching, compatible with the information to be displayed by the LCD. The alignment process is most easily carried out by solution casting (spin coating, roller coating, dipping, spraying, printing and/or doctor blading) an organic polymer onto the two ITO coated substrates. After removal of the solvents and/or curing of the polymer layers, the substrates are usually rubbed or buffed in one direction with cloths. The rubbing process serves to establish a unique optical direction. After rubbing both substrates, they are rotated from 70 to 360 degrees with respect to each other; adhered together using organic adhesives to preserve a constant thickness to a space or gap between the substrates; filled with various mixtures of liquid crystal materials; and finally sealed using organic adhesives. At this stage, polarizing films are often attached to the outside surfaces of the substrates by a lamination process. Finally, electrical connections are made to both substrates in a manner consistent with the electrical and display designs.
The use of rubbed polymer films, i.e. alignment direction and tilt angle controlling films, dominates the process technology used in the production of all categories of liquid crystal displays, and polyimides are the most common alignment films in use today. Moreover, the tilt angle and its magnitude are very important in the various electro-optic responses and the electro-optic properties of the LCD device. The stability, legibility and reliability of the LCD are all related to the magnitude and stability of the tilt angle. The tilt angle has to be stable to high temperature and illumination, and the magnitude of the tilt angle has to be stable for long storage times in order to provide a long operational time for the displays. This holds particularly for the value of the tilt angle obtained after the heat treatment of the display after sealing the cells filled with liquid crystals.
Polyimide films used to control the alignment direction and the tilt angle of liquid crystal molecules in liquid crystal displays are very thin, generally being on the order of from 100 to 2000 angstroms. The alignment is induced in a unique direction of the polyimide polymer by gentle buffing with specific cloths. The actual tilt angle obtained is a function of polymer ordering on the surface, the resulting surface energy, the nature of the cloth used to buff the surface and the amount of buffing work. In addition to these variables, each of the hundreds of commercial liquid crystal formulations interacts differently with a given surface. In general, however, the single most important factor determining the value range of the tilt angle is the intrinsic character of the polymer used to control this angle.
For most standard TN or STN liquid crystal displays (off=white), conventional polyimides based on pyromellitic dianhydride and 4,4'-diaminodiphenyl ether or 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 4,4'-diaminodiphenyl ether and m-phenylene diamine give tilt angles ranging from 2 to 3 degrees which are adequate for these displays. For more sophisticated STN displays, however, higher tilt angles of greater than 4 degrees are required and polyimides modified with alkyl or fluoroalkyl groups have generally been used. There are, however, other liquid crystal display applications which require lower tilt angles of less than 2 degrees while maintaining good and stable alignment properties. For example, normally black liquid crystal displays (off=black) preferably require tilt angles of around 1.5 degrees which are lower than that achieved using conventional polyimides. Tilt angles of less than 1 degree in TN LC displays cause the occurrence of reverse tilt domains, which leads to the scattering of light, reducing the contrast and disturbing the display performance of "normally black" TN LC displays. "Normally black" TN means that the absorption axes of the polarizers of the LCD are essentially parallel to each other and also parallel to one of the two directional orientations of the LC at one substrate side. In contrast to tilt angles of less than 1 degree, tilt angles significantly greater than 2 degrees cause reduced contrast, i.e. an appreciable "off state" transmission due to the reduction of the effective birefringence and cause an increased viewing angle dependence of the contrast. Thus, neither tilt angles of less than 1 degree nor of more than 2 degrees are desirable. Generally, non-fluorinated aromatic polyimides provide surface tilt angles in the range of 2 to 3 degrees.
The frame sealant used in the LCD is typically an organic adhesive which is sealable by exposure to UV radiation or to elevated temperature. Commonly used temperature curable organic adhesives are set at a typical peak temperature of 150.degree. to 200.degree. C. for a curing time ranging from several minutes to several hours.
The alignment of the liquid crystals, which is induced by the rubbing of the alignment layer, has to withstand this temperature treatment, as the sealant has to be cured after rubbing of the surface before filling the cell with the liquid crystal and annealing the LCD. The requirement of stability against heat treatment is most severe for orientation layers which are cured in the same range of temperatures used to set the frame sealant.
Therefore, a need exists for a polyimide alignment film which provides tilt angles in the range of from 1 to 2 degrees, preferably from 1.2 to 1.8 degrees, and most preferably from 1.4 to 1.6 degrees, and also gives good and stable alignment of liquid crystal molecules. These tilt angles have to be stable under the elevated temperatures or illumination (e.g. UV radiation) necessary to cure the sealant materials. They also should be stable to long time storage at elevated temperatures. The tilt angles also have to be stable to changes in parameters of the production process such as curing temperatures and rubbing strength. Also the variation of the tilt angle with the type of liquid crystal mixture used has to be small.
Japanese patent applications 2-309322 and 2-309323, published on Dec. 25, 1990, disclose polyimide alignment films, which reduce image blurring when used in a nematic liquid crystal display element. The polyimide films are derived from a tetracarboxylic dianhydride component such as pyromellitic dianhydride, and an aromatic diamine component containing structural units of the formula ##STR1## wherein m is 0 to 2 used in combination with either (1) a phenylene, biphenylene or terphenylene diamine or (2) a diaminodiphenylsulfone. There is no disclosure, however, of the specific polyimide alignment films of the present invention, which are derived from pyromellitic dianhydride and 1,4-bis or 1,3-bis-(4-aminophenoxy)-benzene, 4,4'-bis(4-aminophenoxy)biphenyl or 4,4'-bis(4-aminophenoxy)diphenylsulfone and which provide tilt angles of from 1 to 2 degrees and give good liquid crystal alignment.